The embodiments disclosed herein relate generally to energy sources with less grid coupled rotating inertia, or other forms of stored energy, than conventional power generators and, more specifically, to providing stabilization control for such non-conventional energy sources. Non-conventional energy sources are variable energy sources that do not generate power at nominal system frequency (e.g., 60 Hz) and are therefore coupled to the grid using fast acting power electronics and controls.
Wind-turbine generators (WTGs) and wind-plants are typically designed to deliver constant active and reactive power to a utility grid with the delivered power being independent of system frequency. This is accomplished by decoupling the rotor inertia and speed from the grid using fast acting power electronics and controls. Due to increases in wind-plant size and penetration, some utilities are now requiring that wind-plant and wind-turbine controls provide enhanced capabilities such as frequency stabilization.
Conventional synchronous generators naturally respond to frequency disturbances due to the grid coupled rotating inertia and governor controls of such generators. Some utility operators require that wind turbines respond in a similar way to frequency disturbances. Specifically, a short duration power increase (for example, five percent of rated power) may be needed when frequency dips below a threshold.
It is a well-known characteristic of utility systems that the grid frequency tends to decrease when the load exceeds the generation and to increase when the generation exceeds the load. Such decreases or increases may occur in a monotonic manner, an oscillating manner, or combinations thereof when the grid is subjected to a sudden change in the balance between generation and load. It is a consideration in the design of such a system that any method to achieve compensation of such decreases or increases should be one that does not cause unacceptable coupling between grid oscillatory modes and the wind turbine mechanical oscillatory modes.
Modern wind plants include the capability to curtail output power below the level available based on wind conditions. Utility grid operators sometimes require curtailment if the available grid power is not needed by the utility. Continuous curtailment may also be required by utility operators to provide operating range for the wind plant to increase power output when frequency decreases. However, curtailment has the undesired consequence of reducing revenues produced by the wind plant.